1. Field of the Invention
This invention relates generally to the field of integrated circuit manufacturing and, more particularly, to ion implantation.
2. Description of the Related Art
Ion implantation is a well known technique for forming junctions in semiconductors. During ion implantation, a source material is used to produce ions which are ultimately implanted into a silicon wafer. Where the ions become embedded in the silicon, they change the silicon's electrical properties. If an N-type source material is used (e.g., one containing phosphorous), the resulting silicon will become N-type, or rich with electrons. If a P-type source material is used (e.g., one containing boron), the resulting silicon will become P-type, or rich with "holes."
P-type junctions have traditionally been manufactured by the use of a boron-based source gas, usually boron trifluoride, or BF.sub.3. Inside of an ion implantation machine, the source gas is ionized, creating, amongst other species, BF.sub.2.sup.+ ions. In traditional Ultra Large Scale Integration (ULSI) applications requiring P-type junctions, the BF.sub.2.sup.+ species is used as the implant species in the formation of boron-doped, P-type junctions. The BF.sub.2.sup.+ species can be extracted by adjusting the field strength of extractor magnet inside of the ion implanter so as to choose an ion species of the desired charge-to-mass ratio.
However, using the BF.sub.2.sup.+ ion as the implant species has its drawbacks. Most importantly, molybdenum is usually present in ion implantation machine components and acts as a source of contamination. An ionized molybdenum ion, Mo.sup.++, has approximately the same charge-to-mass ratio as the BF.sub.2.sup.+ species. Therefore, if Mo.sup.++ ions are present in the ion plasma, the magnet will be unable to extract the desired BF.sub.2.sup.+ species without also extracting the Mo.sup.++ species. As a result, any ionized molybdenum will be implanted into the silicon along with the BF.sub.2.sup.+ species. Unfortunately, the presence of molybdenum degrades the performance of the P-type junctions formed, because the presence of molybdenum in the junction will increase the junction's reverse bias leakage. In DRAM technologies, for instance, increased reverse bias leakage degrades refresh characteristics. Junction degradation issues become particularly acute in the ULSI era, which demands shallow junctions of high purity.